A major difficulty in prior art solution mining, especially when the formation mineral is horizontally layered or fractured, is stagnation of process fluids in blind cracks, fissures and extremities of the down-hole cavity. For optimum removal of mineral, it would be desirable to continuously contact the mineral surfaces with fresh solvent. Where a large portion of the cavity surface consists of the walls of dead-end fissures, it has been substantially impossible to avoid accumulation, in the fissures, of semi-saturated and stagnant solvent. In most prior art processes, such as the Frasch sulfur process, mineral is removed from only those formation surfaces that are closely adjacent a flowing stream of solvent. The process fluid in blind cracks becomes completely stagnant. The overall common result is early channeling of the formation, that is the development of undesirably large passages extending directly from the injection well to the production well and through which the process fluid circulates with an ever-decreasing mineral yield. In solution mining of nahcolite, the problem is compounded in that most of the nahcolite is found in predominantly horizontal stringers and horizontal beds of nodules embedded in a substantially impervious shale formation. While rubblizing has been used with some success as a means for pre-enabling solution mining, it merely alleviates the difficulty and does not enable an intimate non-stagnant contact between the mineral and the process fluids.
A primary principle of our invention is repeated liquid-to-vapor phase change of a solvent or a vaporizable component of a solvent mixture disposed in contact with in-situ minerals which are to be dissolved and extracted in solute and/or slurry form. The invention itself, its advantages, and the details for its optimum application in various environments will be best understood by considering the following exemplary implementations. It is to be understood, however, that our invention is not limited to these examples or any specific application, but may be used for the extraction of substantially any mineral that is soluble in an available solvent capable of being cyclically vaporized and condensed in response to controlled pressure variation over a predetermined pressure range. Alternatively, it is possible to use a solvent mixture containing a substance which can be cyclically evaporated and condensed or evolved (outgassed) and absorbed.
In the past, underground minerals, such as oil shale deposits, were mined and brought to the surface for further processing of the various components and constituents. Underground mining is expensive, time-consuming, and dangerous. Open pit mining of many mineral deposits is prohibited, economically, by thick overburden. In addition, ecological problems add to the costs associated with these methods of extraction.
One technique for in-situ processing of shale oil involves underground tunneling into the shale-oil deposits in a predetermined pattern for the purpose of blasting and rubblizing the deposit. After the deposit is rubblized, a flame front is instituted which causes an in-situ retorting of the hydrocarbon values in the shale. This process has met with limited success primarily because of difficulty in obtaining uniform rubble in the shale deposit with the attending problems of maintaining a reasonably uniform flame front and avoiding large plastic deformation and flow of the oil shale. If the rubble is not reasonably uniform, a substantially uniform flame front is not maintained, and the flames are quenched by the retorting products, or by-pass burning occurs. The plastic flow problems are particularly severe in kerogen-rich deposits.
Pressure fluctuation has been used in the past to improve recovery from conventional oil fields. In one process, steam pressure is cyclically varied (huff and puff) to recover viscous oil from sand and gravel. This prior art technique employs a pressure range such that condensation at the ambient temperature is avoided. Moreover, in the prior art, that technique normally is used only until the heated subsurface of two adjacent wells come into contact, and then it is replaced by continuous steam pressure drive. Moreover, the formations wherein "huff and puff" has been applied are essentially a mixture of heavy oil, sand, and gravel. They have neither the prominant horizontal layered structure nor the blind cracks commonly found in mineral deposits having economically attractive concentrations of nahcolite, dawsonite, or kerogen.